The present invention relates generally to implantable cardiac stimulation devices, and more particularly to an automatic pacing stimulus capture assurance system and related method for use in an implantable pacemaker or defibrillator.
Pacemakers are used to treat a condition called bradycardia, in which the heart beats too slowly. A pacemaker system includes three componentsxe2x80x94a pulse generator, at least one pacing lead, and a programmer. The pulse generator contains the battery and the electronic circuitry, or xe2x80x9cbrain,xe2x80x9d which directs the pulse generator to send electrical stimulation pulses through the pacing leads, stimulating the heart and causing it to beat at a controlled xe2x80x9cnormalxe2x80x9d rhythm. The pacing leads may also be used to transmit cardiac signals (i.e., depolarization signals) to the pulse generator. Pacemakers may also include physiological sensors which provide the pacemaker with an indicia of patient hemodynamic needs, so that the pacemaker can adjust its pacing strategy to satisfy those needs. An external programmer is used to monitor the operation of the pacemaker noninvasively (referred to as interrogating the pacemaker) and to change pacemaker settings (referred to as programming the pacemaker).
Implantable cardioverter/defibrillators (ICD""s) are used to treat a condition called tachycardia, in which the heart beats at a rapid, uncoordinated manner. An ICD system, like a pacemaker system, is made up of three componentsxe2x80x94an ICD pulse generator, at least one lead, and a programmer used to interrogate and program the ICD. The ICD pulse generator monitors the rhythm of the heart from the leads, and administers an electric shock when necessary to control/terminate tachycardias and restore a normal heartbeat. ICD""s also include pacemakers, since many patients needing an ICD can generally benefit from some pacing therapy.
Pacemaker technology has evolved rapidly over the last several decades, resulting in improvements making pacemakers more automatic in their ability to adapt to the specific needs of individual patients. ICD""s have also evolved to become increasingly sophisticated both in their treatment of tachycardias and in their inclusion of full-featured pacemakers. In addition, both pacemakers and ICD""s have become significantly smaller with greater longevity resulting from increasingly efficient operation which conserves battery power. It is certainly widely appreciated by those skilled in the art that it is imperative to minimize the operating current required by these devices to achieve the twin goals of extending their operating lives and making the device size as small as is possible.
One of the most significant ways of increasing the efficiency of pacemakers, as well as the pacemaker system operation in ICD""s, has been the development of systems which automatically and continuously adjust the level of energy of electrical stimulation pulses delivered to pace the patient""s heart. In order for a stimulation pulse from a pacemaker to depolarize cardiac muscle tissue to cause it to contract (to cause a heartbeat), the energy of the stimulation pulse must be sufficient to xe2x80x9ccapturexe2x80x9d the heart, that is to cause a depolarization of the atrium or ventricle. The level of energy in a pacemaker stimulation pulse which is necessary to cause capture is referred to as the stimulation threshold level, or simply as the stimulation threshold. Most pacemakers and ICD""s are capable of determining the stimulation threshold in a test which reduces stimulation pulse energy until loss of capture is detected.
If the stimulation pulse is below the stimulation threshold, capture will not occur and the stimulation pulse will be ineffective. If, on the other hand, the stimulation pulse is at or above the stimulation threshold, capture will most likely occur. The amount of energy in the pacemaker stimulation pulse above the stimulation threshold provides no useful function and is wasted. For the purposes of conserving battery energy, and maximizing the life of the device, it is desirable to keep the amount of energy in the pacemaker stimulation pulse or slightly above at the stimulation threshold.
The stimulation threshold varies widely, not only from patient to patient but for any given patient substantially over both longer and shorter periods of time. Eating and sleeping can cause about a twenty percent increase in the stimulation threshold. Posture and exercise can change the stimulation threshold about fifteen to twenty percent. Following lead implantation, the stimulation threshold typically increases to a peak level three months after implantation, and then stabilizes at a lower level.
Since it is desirable to ensure capture, physicians typically program the pacemaker or ICD to deliver pacemaker stimulation pulses at an energy level substantially above the stimulation threshold. The amount that the pacemaker stimulation pulse energy exceeds the stimulation threshold is referred to as the xe2x80x9csafety factor.xe2x80x9d Physicians generally program the stimulation pulse energy at a safety factor of 1.7 to 2 times the stimulation threshold. It is well appreciated by those skilled in the art that the safety factor results in a substantially increased level of current drain from the battery, and in reduced device longevity. Since patient safety is paramount, this has been a situation which, until a few years ago, was acceptable.
Recently, the first pacemakers with automatic capture confirmation on a beat by beat basis were developed. They combine automatic backup pulse delivery upon loss of capture with automatic output regulation of stimulation pulse levels to a value just above the stimulation threshold. These devices automatically search for and locate the stimulation threshold and pace at a level just above that stimulation threshold (for example, 0.3 volts above the stimulation threshold), typically with the amplitude or voltage level of the stimulation pulse being varied and the pulse width remaining constant, such as, for example, at a nominal value of 0.5 mS. Such a stimulation threshold search may be automatically done periodically, such as, for example, every eight hours.
These devices also monitor capture confirmation by looking for an evoked response during a window of time immediately after the stimulation pulse; for example, the window may begin 15 mS after each stimulation pulse, with the width of the window being 47.5 mS. Finally, these devices assume loss of capture if no evoked response is sensed during the window after the stimulation pulse, and, in the absence of an evoked response signal, provide a higher voltage (typically 4.5 volts, for example) safety backup pulse. Thus, the device ensures that the patient""s heart never misses a beat. In the event of loss of capture indicating a change in the stimulation threshold (which may be presumed, for example, after the delivery of two consecutive backup pulses), the device will search again for the stimulation threshold.
One example of a system for locating the stimulation threshold is shown in U.S. Pat. No. 5,669,392, to Ljungstrxc3x6m. Similarly, an example of a capture confirmation system is illustrated in U.S. Pat. No. 5,782,889, to Hxc3x6egnelid et al. Finally, an example of a system for automatically providing a backup pulse is shown in U.S. Pat. No. 5,846,264, to Andersson et al., all of which are each hereby incorporated herein by reference.
Another automatic system having similar characteristics is described in U.S. Pat. No. 5,350,410, to Kleks et al. is hereby incorporated herein by reference. It may also be noted that these features may also be included in the pacemaker contained in an ICD.
The systems described above thus represent a significant enhancement to pacemaker system longevity through minimization of current drain by operating at much lower safety factors. However, they automatically vary the amplitude or voltage of the stimulation pulse without looking at or taking into account the actual variation in current drain caused by a change in the voltage level of the stimulation pulse. Since capture is a function of energy delivered to the cardiac tissue, a more realistic manner of considering stimulation threshold is to view it as a continuous function described by the strength-duration relationship.
See, for example, Stokes and Bornzin, xe2x80x9cThe Electrode-Biointerface: Stimulationxe2x80x9d, Chapter 3 of Modern Cardiac Pacing, edited by S. Serge Barold, M.D. (Futura Publishing Co., 1985). The fundamental nature of the strength-duration relationship is that for very narrow pulse widths, a large stimulation pulse amplitude is required to obtain capture, and for wide pulse widths, a lower stimulation pulse amplitude is required to obtain capture. Thus, there is a wide variety of pulse amplitude and pulse width combinations which may be used to obtain capture at a given stimulation threshold. There are also a corresponding wide variety of pulse amplitudes and pulse widths combinations which may be used to provide an adequate safety factor.
However, the ability to independently program pulse amplitude and pulse width does not necessarily provide optimal pacing, because in order to maintain a desired or adequate safety factor, battery current drain is not considered as a primary factor. The first to note this was Bornzin in U.S. Pat. No. 5,697,956 (the ""956 patent), which is assigned to the assignee of the present invention. Bornzin did not provide independent programming of pulse amplitude and pulse width, but rather provided only the programming of pulse energy, selected from a series of pulses of increasing or decreasing energy that had been selected to provide optimal pacing. This basic principle is the foundation of the present invention as well, and U.S. Pat. No. 5,697,956 is hereby incorporated herein by reference.
The technique taught in the ""956 patent is a five step procedure which first determines the patient""s strength-duration curve by determining a series of pulse thresholds for a particular patient as a function of a plurality of pulse widths. Second, desired pulse amplitude safety factor is added to the strength-duration curve to produce a plurality of pulse amplitude and pulse width combinations that would ensure capture at the desired safety factor. This may be thought of as a second curve identical to the strength-duration curve but displaced directly above it by an amount equal to the increase in voltage selected as a safety factor.
Third, a pulse current drain is computed as a function of each of the pulse amplitude and pulse width combinations determined in the second step. Fourth, a series of optimal pulse amplitude and pulse width combinations is selected that provides a minimal pacing current drain as a function of pacing energy. Fifth, the pacemaker is automatically programmed to the optimal pacing energy using the pulse amplitude and pulse width combination determined in the fourth step.
The procedure taught by the ""956 patent represented a significant improvement over the previously used technique of manually programming the pacemaker to a desired pulse width, determining the voltage necessary to assure capture, and programming a higher voltage to provide a safety factor. For the first time, the pacing current required by the device was used as a guide to determine the pulse amplitude and the pulse width used. However, the ""956 patent does not provide a method of optimization of such a device by the use of an automatic capture confirmation system. Nor does the system of the ""956 patent truly determine optimal operating points, but rather it finds optimal points and adds a pulse amplitude (voltage) increment to such points which however, does not result in establishing optimal operating points.
It is a primary feature of the present invention to determine improved or optimal operating points from a battery current efficiency standpoint, and to operate the device at such operating points. In this regard, it is an objective of the present invention to determine those pulse amplitude/pulse width combinations that have the lowest battery drain of such combinations.
Briefly, the present invention determines improved or optimal operating points (stimulation pulse amplitude and stimulation pulse width combinations, hereinafter pulse amplitude/pulse width combinations) from an efficiency standpoint, as measured by battery charge drained from the battery. These operating points vary widely in the pacing energy that they deliver to cardiac tissue, and, as such, offer a complete array of pulse amplitude/pulse width combinations suitable to meeting the pacing thresholds of virtually any patient. Following the determination of these operating points, the pacing system is operated such that it paces at these operating points.
In one embodiment, the determination of the operating points is made by determining which of all of the possible pulse amplitude/pulse width combinations have the lowest battery charge drain at a predetermined level of stimulation efficacy. The level of stimulation efficacy required varies from patient to patient, and will also vary in the same patient over a period of time. The objective is thus to determine and utilize a quantifiable measure of stimulation efficacy.